Method for coating articles with pyrolitic graphite



Sept. 14, 1965 $206,331

METHOD FOR COATING ARTICLES WITH PYROLY'I'IC GRAPHITE R. J. DI EFENDORF Filed April 25 1961 [raven-6:07: Russe J D/ef'endor'fi b W Wmzmr His Attorney.

United States Patent 3,206,331 METHOD FOR COATING ARTICLES WITH PYROL'ITIC GRAPHITE Russell J. Diefendorf, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 25, 1961, Ser. No. 105,493 Claims. (Cl. 117-226) This invention relates to methods of forming articles and more particularly to methods of forming pyrolytic graphite articles.

This application is a continuation-in-part of my copending application filed December 12, 1960, as Serial Number 75,244 and now abandoned.

Pyrolytio graphite is defined as a material made from carbonaceous gases by thermal decomposition or from a carbonaceous material by evaporation and deposition on a surface. In pyrolytic graphite, planar graphite crystallites are arranged so that their layer structures are parallel to the deposition surface. It is useful as a high temperature material for lamp filaments, furnace linings and neutron reactor moderators. Development of missile and space propulsion systems has created an additional requirement for pyrolytic graphite components in these systems.

Carbonaceous gases have been thermally decomposed and deposited on a surface to produce pyrolytic graphite. As a result of the decomposition, carbon is removed from the gas and deposits on the surface so that planar graphite crystallites are aligned into a layer structure. The grain structure of pyrolytic graphite articles is determined by the condition of the surface or member upon which the article is deposited. When commercial graphite is employed as the deposition surface or member, iron and other impurities boil to its surface during deposition of pyrolytic graphite thereon. The iron and other impurities create large grains in the deposited pyrolytic graphite. Therefore, it would be desirable to provide deposition methods of forming pyrolytic graphite articles with finegrain structure.

It is an object of my invention to provide a deposition method of forming pyrolytic graphite articles.

It is a further object of my invention to provide a deposition method of forming pyrolytic graphite articles with finer grain structure.

It is a still further object of my invention to provide a deposition method of forming pyrolytic graphite articles in which iron and other impurities are removed from the deposition surface.

In carrying out my invention in one form, a deposition method comprises providing an enclosure, positioning a member within the enclosure and spaced therefrom to provide a narrow passage therebetween, evacuating the passage, heating the member and enclosure to a temperature of at least 2350 C. to remove impurities from the member and the enclosure, maintaining the temperature of the member and the enclosure in the range of 2000 C. to 2500 C., and flowing a carbon vapor through the passage whereby pyrolytic graphite is formed on the member and enclosure.

These and various other objects, features, and advantages of the invention will be better understood from the following description taken in connection with the accompanying drawing in which:

The single figure is a sectional view of a deposition apparatus embodying my invention.

In the single figure of the drawing, a deposition apparatus is shown generally at 10 which comprises a chamber 11 having a lower body portion 12 and a cover 13 which is hinged to the lower body portion by means of bolts 14 and employs an O ring 15 therebetween. Viewing window 16 is provided in cover portion 13 to view the operation and to read an optical pyrometer (not shown). A preheater 17 is positioned on the inner surface of the bottom wall of chamber 11 and consists of a container 18 having an inlet 19 and outlet 20. A bafiie 21 is positioned within the preheater and is provided with a pluarlity of openings 22 around the perimeter thereof.

A feed line 23 is connected at the inlet opening 19 of preheater 17 and extends through the bottom wall of chamber 12 to a carbonaceous material source (not shown). A carbonaceous gas is fed from the source through a meter 24 showing the total consumption of gas, a gas rate meter 25, an acetone and Dry Ice trap indicated at 26, and line 23 to preheater 17. While a pure carbonaceous gas, such as methane, ethane, propane, acetylene, benzene, carbon tetrachloride, or cyanogen, is employed, the carbonaceous material can also be in liquid or solid form which is fed from the source to preheater for conversion to a carbon vapor. Such a carbonaceous gas can be mixed with a non-carbonaceous gas selected from the group consisting of hydrogen and nitrogen which reacts with the carbonaceous gas during its decomposition to carbon. For example, hydrogen, as the reacting gas, can be employed with the alkanes or alkynes while nitrogen, as the reacting gas, can be used with cyanogen.

An enclosure 27 of graphite or other high temperature material having an inlet 28 and an outlet 29 is positioned on preheater 17 by aligning inlet 28 of the enclosure with outlet 20 of preheater 17. Enclosure 27 can be constructed of several pieces, such as a lower portion 30 and an upper portion 31 joined together. In this manner, a member 32 is placed Within the enclosure and supported therein in a relatively simple fashion. There is shown a member 32 of graphite or other high temperature material supported concentrically within enclosure 27 by means of pins 33 which extend through member 32 and through the walls of enclosure 27. Additionally, nuts 34 have been applied on the pins 33 adjacent to the exterior surface of member 32 to hold the mandrel in position within enclosure 27. I found also that the use of reentrant angles on the member allows the pyrolytic graphite article formed thereon to be easily separated from the member. Member 32 and enclosure 27 form a narrow passage 35 between the exterior surface of member 32 and the interior surface of enclosure 27. I prefer to employ a uniform diameter passage or a passage narrowing toward its outlet to produce a more uniform deposition. A chimney 36 surrounds outlet 29 of enclosure 27 to provide for removal of fumes which pass through passage 35 between member 32 and the interior wall of enclosure 27. Suitable insulation in the form of carbon black 37 surrounds enclosure 27 and is held in position by a quartz or asbestos paper cylinder 38. Conventional induction heating coils 39 surround cylinder 38 to provide heat to remove impurities from member 32 and heat for preheater 17, enclosure 27, member 32, and passage 35 during the deposition process. Chamber 12 is also provided with an outlet 40 to which is connected a line 41 associated with a vacuum pump 42 to reduce the pressure in chamber 12.

While it is disclosed that the enclosure is a two-piece structure, a one-piece structure or a plurality of pieces can be employed. If a plurality of pieces are used, the interior surface of the enclosure can be machined to conform to the exterior contour of the mandrel. Additionally, the chimney can be supported from the interior wall of lower body portion 12 or cover 13. If desired, the mandrel support can be extended through the enclosure inlet.

I discovered unexpectedly that finer grain pyrolytic graphite articles were deposited uniformly and without soot particles at high volume carbonaceous gas flow rates by positioning concentrically a member within an enclosure, spacing the member from the enclosure to provide a narrow passage therebetween, evacuating the passage, heating the member and the enclosure to a temperature of at least 2350 C. to remove impurities from the member and the enclosure, maintaining the temperature of the member and the enclosure in the range of 2000 C., to 2500 C., and flowing a carbon vapor through the passage. I found further that a fixed diameter passage should have a diameter of at least twice the deposition thickness since both the member and enclosure wall are coated in the process. The passage diameter is also determined by the member diameter and passage length to provide a negligible pressure drop. The concentration gradient of carbon to be deposited should also be small.

The grain structure of pyrolytic graphite is determined by the condition of the surface or member upon which the article is deposited. When commercial graphite is employed as the deposition surface or member, iron and other impurities boil to its surface during deposition of pyrolytic graphite thereon. The iron and other impurities create large grains in the deposited pyrolytic graphite. I found that finer grain articles with resulting higher strength could be produced by heating the deposition member and its associated enclosure to a temperature of at least 2350 C. while the passage therebetween is evacuated, preferably to the lowest obtainable vacuum, prior to flowing a carbon vapor through the passage. While the time of heating can be varied, it is easily ascertained for a particular deposition surface since the pressure rises during treatment and falls thereafter.

In the operation of deposition apparatus shown in the single figure, a member 32 is positioned concentrically within upper body portion 31 by means of bolts 33 and nuts 34. Portion 31 is affixed to lower body portion 3%) to form enclosure 27. Member 32 and enclosure 27 are spaced apart to provide a narrow, substantially uniform diameter passage therebetween. Since the narrow passage diameter will befixed, it is determined by the member diameter and passage length to provide a negligible pressure drop and by a requirement for a diameter at least twice the thickness of the proposed member deposition. Enclosure 27 is then positioned on preheater 17 with outlet 20 of preheater 17 and inlet 28 of enclosure 27 in alignment. Chimney 36 is placed over outlet 29. Cylinder 38 surrounds enclosure 27 and provides a space which is filled with carbon black insulation. An induction coil 39 surrounds cylinder 38 for heating preheater 17, enclosure 27, member 32, and passage 35. Cover 13 is bolted to lower body portion 12 of chamber 11.

The chamber atmosphere is reduced preferably to the lowest obtainable vacuum. Power is supplied to induction coil 39 which heats preheater 1'7, enclosure 27, mandrel 32, and passage 35 to a temperature of at least 2350 C. to purify member 32 and the interior wall of enclosure 27. During heating, the pressure rises while upon completion, the pressure falls to its initial value. This heating step removes iron and other impurities which boil to the surfaces of the member and enclosure and adsorbed gases which are present. I have found that the employment of this heating step provides member 32 with a surface on which finer grain and high strength pyrolytic graphite is formed. The power is then shut OE and the assembly within the chamber 11 is allowed to cool. I prefer generally to bring the chamber to atmospheric pressure and open the chamber to inspect the mandrel and enclosure prior to forming pyrolytic graphite articles. Any carbon black present on the surfaces is removed manusually. It is not necessary to shut off the power, cool the assembly, open and inspect the chamber since a carbon vapor can be flowed through the enclosure passage subsequent to the purification of the deposition surface to form pyrolytic graphite. After such an inspection, cover 13 is bolted again to lower body portion 12, and chamber 11 atmosphere is reduced preferably to the lowest obtainable vacuum.

A carbonaceous gas, such as methane, is fed through a total consumption meter 24, a gas rate meter 25, and an acetone and Dry Ice trap 26 prior to entering preheater 17 through gas line 23. Power is supplied to induction coil 39 to bring the temperature of preheater 17, enclosure 27, member 32 and passage 35 up to a temperature in the range of 2000 C. to 2500 C. I have found that this temperature range is desirable to produce uniform pyrolytic graphite articles. Since density is desirable to provide a free-standing body which can be removed readily from the mandrel. The carbonaceous gas is preheated in preheater 17 at a temperature in the above temperature range whereby a carbon vapor is formed. If a liquid or solid carbonaceous material is used, the material is fed to preheater 17 in which it is converted to its gaseous form and then to a carbon vapor.

While I prefer to use induction coil 39 to heat preheater 17, enclosure 27, member 32, and passage 35, the carbonaceous gas can be preheated from a separate heat source to the desired temperature to provide a carbon vapor which flows through passage 35. Additionally, the carbonaceous gas can be fed to passage 35 where heat is supplied to enclosure 27, member 32, and passage 35 to decompose the gas to a carbon vapor. However, I have found it most desirable to preheat the gas in preheater 17 and feed the carbon vapor through passage 35 while enclosure 27, member 32, and passage 35 are heated to maintain the temperature of the vapor in the passage. Although some of the carbon vapor deposits on the walls of the preheater, most of the vapor is deposited on the enclosure and the mandrel. I have found also that the most beneficial results are secured from employing additionally a narrow, uniform diameter passage or a passage narrowing toward its outlet. The preheating step and uniform or tapering passage tend to maintain a uniform coating thickness.

The above process with a carbonaceous gas can be carried out over a wide range of flow conditions, such as 0.5 mm. to 760 mm. of mercury, at various gas flow rates, such as 20 to cubic feet per hour which is similar to molecular flow. The addition of a reacting gas, such as hydrogen, with the carbonaceous gas creates a flow condition which is also similar to molecular flow in that the reacting gas slows down growth of the carbon particles to provide more time for diffusion to the wall prior to attaining critical size for soot formation. Generally, a ratio of at least one to one of reacting gas to carbonaceous gas is employed. In the course of my research on uniform deposition at an increased rate which disclosed that such deposition depends upon a number of variables which were mentioned above, I have found, also unexpectedly, that the highest rate of deposition occurs immediately prior to a sooting environment. Accordingly, the carbon vapor flow rate is increased to produce a sooting environment which is observed through window 16 in cover 13 of the apparatus. Thereafter, the vapor flow is reduced below sooting environment and the flow continued to form pyrolytic graphite on the member and enclosure. After the desired thickness of the pyrolytic graphite article is attained, the gas flow is stopped, and the assembly within chamber 11 is allowed to cool to room temperature. The pressure is increased subsequently to atmospheric pressure, and cover 13 is removed to provide access to enclosure 27.

Portion 31 .is detached from portion 30, and member 32 removed from portion 31. With the particular type of member 32 employed in the single figure, the coated number is cut in two or the article is cut at approximately its midpoint to remove the pyrolytic graphite articles therefrom. Additionally, re-entrant angles can be employed to provide for easy separation of the article from the member. The member can also be constructed of several pieces and include a partial central bore which causes the member to collapse upon removal of the article.

During the operation of forming such articles, the temperature is recorded by an optical pyrometer (not shown) which is viewed through window 16 in cover 13.

Temperatures in the range of 2000 C. to 2500 C. can be employed to produce articles on member 32. At such a temperature, it is possible to form 20 to 40 mils of pyrolytic graphite an hour to produce articles having a wall thickness of 100 to 200 mils without soot inclusions.

If it is desired to provide a thicker deposition on a portion of member 32, the mandrel can be eccentrically positioned within enclosure 27. In this manner, a pyrolytic graphite article can be formed with a wall of varying thickness.

Additionally, pyrolytic graphite sheets are formed in a similar manner by employing members in sheet form. A plurality of these sheets are positioned within an enclosure and spaced apart to provide a passage between adjacent pairs of sheets. A passage can also be provided between the enclosure and the members. Pyrolytic graphite is formed on these sheets in the same manner as described above for forming an article on member 32. The pyrolytic graphite article is subsequently removed from each of the sheet members.

Several examples of methods of forming pyrolytic graphite articles in accordance with the present invention are as follows:

Example I A deposition apparatus was set up generally in accordance with the drawing wherein both the enclosure and the member were composed of commercial graphite to form an annular passage which narrowed from inch to /2 inch toward the outlet. The member which included re-entrant angles, a removable bottom portion, and a central bore, had a maximum diameter of 2% inches, and a length of 4 inches. After the cover was bolted to the lower body portion, the chamber atmosphere was reduced to a pressure of .001 mm. of mercury by the pump. Power was supplied to the induction coil to heat the member, enclosure, and passage to an uncorrected optical pyrometer temperature reading of about 2285 C. During heating the pressure rose. After removal of iron, other impurities and adsorbed gases, the pressure fell. The power was discontinued, and the deposition apparatus was allowed to cool to room temperature. The chamber was then opened, inspected, and closed. The chamber atmosphere was again reduced to a pressure of .001 mm. of mercury. Power was supplied to the induction coil to heat the enclosure, member, passage, and preheater to an uncorrected optical pyrometer temperature reading of 2285 C. A carbonaceous gas in the form of methane was supplied at a rate of 36 cubic feet per hour at a pressure of 1140 mm. of mercury to the preheater subsequent to flowing through metering devices, and an acetone and Dry Ice bath. The gas formed into a carbon vapor in the preheater which was deposited uniformly on both the member and interior enclosure wall as it flowed through the narrow passage at a pressure of approximately 18 mm. of mercury. After live hours, the power and gas fiow were discontinued and the chamber was restored to atmospheric pressure. After cooling to room temperature, the member was removed from the enclosure. The pyrolytic graphite article was removed from the member. The article had a thickness of 180 mils.

Example II A deposition apparatus was set up generally in accordance with the drawing wherein both the enclosure and the member were composed of commercial graphite to form an annular passage which narrowed from 4 inch to /2 inch toward the outlet. The member, which included re-entrant angles, a removable bottom portion, and a central bore, had a maximum diameter of 2% inches, and a length of 4 /2 inches. After the cover was bolted to the lower body portion, the chamber atmosphere was reduced to a pressure of .001 mm. of mercury. Power was supplied to the induction coil to heat the member, enclosure, and passage to an uncorrected opti cal pyrometer temperature reading of about 2285 C. During heating, the pressure rose. After removal of iron, other impurities and adsorbed gases, the pressure fell. The power was discontinued, and the deposition apparatus was allowed to cool to room temperature. The chamber was then opened, inspected, and closed. The chamber atmosphere was again reduced to a pressure of .001 mm. of mercury. Power was supplied to the induction coil to heat the enclosure, member, passage, and preheater to an uncorrected optical pyrometer temperature reading of 2285" C. A carbonaceous gas in the form of methane was supplied to the preheater subsequent to flowing through metering devices, and an acetone and Dry Ice bath to form a carbon vapor. The carbon vapor flow was increased to 38 cubic feet per hour to produce a sooting environment which was viewed through the cover window. The flow was then reduced to 36 cubic feet per hour which was below the sooting environment. The carbon vapor was deposited on both the member and interior enclosure wall as it flowed through the passage at a pressure of approximately 18 mm. of mercury. After five hours, the power and gas flow were discontinued and the chamber was restored to atmospheric pressure. After cooling to room temperature, the member was removed from the enclosure. The pyrolytic graphite article was removed from the member. The article had a thickness of 180 mils.

Example 111 A deposition apparatus was set up generally in accordance with the drawing. Three sheets of commercial graphite having dimensions of 17 inches by 30 inches were spaced apart within an enclosure of commercial graphite. A passage was provided between each pair of sheets. The sheets were positioned so that each passage narrowed from 3 inches width to 1 inch width toward the enclosure outlet. After the cover was bolted to the lower body portion, the chamber atmosphere was reduced to a pressure of .020 mm. of mercury by the pump. Power was supplied to the induction coil to heat the sheets, enclosure, and passages to an uncorrected optical pyrometer temperature reading of about 2130 C. During heating the pressure rose. After removal of iron, other impurities and adsorbed gases, the pressure fell. The power was discontinued, and the deposition apparatus was allowed to cool to room temperature. The chamber was then opened, inspected, and closed. The chamber atmosphere was again reduced to a pressure of .020 mm. of mercury.. Power was supplied to the induction coil to heat the enclosure, members, passage, and preheater to an uncorrected optical pyrometer temperature reading of 2150 C. A mixture of a carbonaceous gas in the form of methane and hydrogen gas at a ratio of one to one parts was supplied at a rate of 60 cubic feet per hour at a pressure of 1140 mm. of mercury to the preheater subsequent to flowing through metering devices, and an acetone and Dry Ice bath. The gas formed into a carbon vapor in the preheater which was deposited on the sheets as it flowed through the passage at a pressure of approximately 6 mm. of mercury. After 85 hours, the power and gas flow were discontinued and the chamber was restored to atmospheric pressure. After cooling to room temperature, the sheets were removed from the enclosure. The pyrolytic graphite article was removed from each sheet and had a thickness of 500' mils.

Example IV sheets were positioned so that each passage narrowed from 1 inch width to inch width toward the enclosure outlet. After the cover was bolted to the lower body portion, the chamber atmosphere was reduced to a pressure of .001 mm. of mercury by the pump. Power was supplied to the induction coil to heat the sheets, enclosure, and passages to an uncorrected optical pyrometer temperature reading of about 2250 C. During heating the pressure rose. After removal of iron, other impurities and adsorbed gases, the pressure tell. The power was discontinued, and the deposition apparatus was allowed to cool to room temperature. The chamber was then opened, inspected, and closed. The chamber atmosphere was again reduced to a pressure of .001 mm. of mercury. Power was supplied to the induction coil to heat the sheets, members, passages, and preheater to an uncorrected optical pyrometer temperature reading of 2300 C. A carbonaceous gas in the form of methane was supplied at a rate of 33.0 cubic feet per hour at a pressure of 1140 mm. of mercury to the preheater subsequent to flowing through metering devices, and an acetone and Dry Ice bath. The gas formed into a carbon vapor in the preheater which was deposited on the sheets as it flowed through the passages at a pressure of approximately 18 mm. of mercury. After three hours, the power and gas flow were discontinued and the chamber was restored to atmospheric pressure. After cooling to room temperature, the sheets were removed from the enclosure. The pyrolytic graphite article was removed from each sheet and had a thickness of 70 mils.

Example V A deposition apparatus was set up generally in accordance with drawing. Three sheets of commercial graphite having dimensions of 3% inches by 4 inches were spaced apart within an enclosure of commercial graphite. A passage was provided between each pair of sheets. The sheets were positioned uniformly so that each passage was inch wide. After the cover was bolted to the lower body portion, the chamber atmosphere was reduced to a pressure of .001 mm. of mercury by the pump. Power was supplied to the induction coil to heat the sheets, enclosure, and passages to an uncorrected optical pyrometer temperature reading of about 2250 C. During heating the pressure rose. After removal of iron, other impurities and adsorbed gases, the pressure fell. The power was discontinued, and the deposition apparatus was allowed to cool to room temperature. The chamber was then opened, inspected, and closed. The chamber atmosphere was again reduced to a pressure of .001 mm. of mercury. Power was supplied to the induction coil to heat the sheets, members, passages, and preheater to an uncorrected optical pyrometer temperature reading of 2280 C. A carbonaceous gas in the form of methane was supplied at a rate of 150 cubic feet per hour at a pressure of 1140 mm. of mercury to the preheater subsequent to flowing through metering devices, and an acetone and Dry Ice bath. The gas formed into a carbon vapor in the preheater which was deposited on both the sheets as it flowed through the passages at a pressure of 20 mm. of mercury. After 3 hours, the power and gas flow were discontinued and the chamber was restored to atmospheric pressure. After cooling to room temperature, the sheets were removed from the enclosure. The pyrolytic graphite article was removed from each sheet and had a thickness of 90 mils.

While other modifications of this invention and variations of method which may employed within the scope of the invention have not been described, the invention is intended to include such that may be embraced within the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A deposition method which comprises providing an enclosure, Positioning a member within said enclosure and spaced therefrom to provide a narrow passage therebetween, evacuating siad passage, heating said member and said enclosure to a temperature of at least 2350 C. to remove impurities from said member and said enclosure, and flowing carbon vapor at a temperature in the range of 2000 C. to 2500 C. through said passage whereby pyrolytic graphite is formed on said member and said enclosure.

2. A deposition method which comprises providing an enclosure, positioning a member within said enclosure and spaced therefrom to provide a narrow passage therebetween, evacuating said passage, heating said member and said enclosure to a temperature of at least 2350 C. to remove impurities from said member and said enclosure, and flowing a mixture of carbon vapor and a reactive gas selected from the group consisting of hydrogen and nitrogen at a temperature in the range of 2000 C. to 2500 C. through said passage whereby pyrolytic graphite is formed on said member and said enclosure.

3. A deposition method which comprises providing an enclosure, positioning a member within said enclosure and spaced therefrom to provide a narrow passage therebetween, evacuating said passage, heating said member and said enclosure to a temperature of at least 2350 C. to remove impurities from said member and said enclosure, maintaining the temperature of said member and said enclosure in the range of 2000 C. to 2500 C., and flowing carbon vapor through said passage whereby pyrolytic graphite is formed on said member and said enclosure.

4. A deposition method which comprises providing an enclosure, positioning a member within said enclosure and spaced therefrom to provide a narrow passage therebetween, evacuating said passage, heating said member and said enclosure to a temperature of at least 2350 C. to remove impurities from said member and said enclosure, feeding a carbonaceous gas to said passage, maintaining the temperature of said member and said enclosure in the range of 2000 C. to 2500 C. to decompose said gas to carbon vapor, and flowing said carbon vapor through said passage whereby pyrolytic graphite is formed on said member and said enclosure.

5. A deposition method which comprises providing an enclosure, positioning a member within said enclosure and spaced therefrom to provide a narrow passage therebetween, evacuating said passage, heating said member and said enclosure to a temperature of at least 2350 C. to remove impurities from said member and said enclosure, flowing carbon vapor at a temperature in the range of 2000 C. to 2500 C. through said passage, increasing said flow to produce a sooting environment, reducing said flow below said sooting environment, and continuing said flow whereby pyrolytic graphite is formed on said member and said enclosure.

6. A deposition method which comprises providing an enclosure, positioning a member within said enclosure and spaced therefrom to provide a passage therebetween, evacuating said passage, heating said member and said enclosure to a temperature of at least 2350 C. to remove impurities from said member and said enclosure, maintaining the temperature of said member and said enclosure in the range of 2000 C. to 2500 C., flowing carbon vapor through said passage, increasing said flow to produce a sooting environment, reducing said flow below said sooting environment, and continuing said flow whereby pyrolytic graphite is formed on said member and said enclosure.

7. A deposition method which comprises providing an enclosure, positioning a plurality of members within said enclosure and spaced therefrom to provide a narrow passage between said members and said enclosure, providing a narrow passage between each pair of adjacent members, evacuating said passages, heating said members and said enclosure to a temperature of at least 2350 C. to remove impurities from said members and said enclosure,

9 and flowing carbon vapor at a temperature in the range of 2000 C. to 2500 C. through said passages whereby pyrolytic graphite is formed on said members and said enclosure.

8. A deposition method which comprises providing an enclosure, positioning a plurality of members within said enclosure and spaced therefrom to provide a narrow passage between said members and said enclosure, providing a narrow passage between each pair of adjacent members, evacuating said passages, heating said members and said enclosure to a temperature of at least 2350 C. to remove impurities from said members and said enclosure, flowing carbon vapor at a temperature in the range of 2000 C. to 2500 C. through said passages, increasing said flow to produce a sooting environment, reducing said flow below sooting environment, and continuing said flow whereby pyrolytic graphite is formed on said members and said enclosure.

9. A deposition method which comprises providing an enclosure, positioning a plurality of members within said enclosure, providing a narrow passage between each pair of adjacent members, evacuating said passages, heating said members and said enclosure to a temperature of at least 2350 C. to remove impurities from said members and said enclosure, and flowing carbon vapor at a temperature in the range of 2000 C. to 2500 C. through said passages whereby pyrolytic graphite is formed on said members.

10. A deposition method which comprises providing an enclosure, positioning a plurality of members within said enclosure, providing a narrow passage between each pair of adjacent members, evacuating said passages, heating said members and said enclosure to a temperature of at least 2350 C. to remove impurities from said members and said enclosure, flowing carbon vapor at a temperature in the range of 2000" C. to 2500 C. through said passages, increasing said flow to produce a sooting environment, reducing said flow below sooting environment, and continuing said flow whereby pyrolytic graphite is formed on said members.

References Cited by the Examiner UNITED STATES PATENTS 2,239,414 4/41 Eddison 117-46 2,817,605 12/57 Sanz 117l06 X 2,922,722 1/60 Hutcheon l17--46 FOREIGN PATENTS 1,022,192 1/58 Germany.

OTHER REFERENCES Holland: Vacuum Deposition of Thin Films, 1956, John Wiley and Sons.

RICHARD D. NEVIUS, Primary Examiner. 

1. A DEPOSITION METHOD WHICH COMPRISES PROVIDING AN ENCLOSURE, POSITIONING A MEMBER WITHIN SAID ENCLOSURE AND SPACED THEREFROM TO PROVIDE A NARROW PASSAGE THEREBETWEEN, EVACUATING SAID PASSAGE, HEATING SAID MEMBER AND SAID ENCLOSURE TO A TEMPERATURE OF AT LEAST 2350*C. TO REMOVE IMPURITIES FROM SAID MEMBER AND SAID ENCLOSURE, AND FLOWING CARBON VAPOR AT A TEMPERATURE IN THE RANGE OF 2000*C. TO 2500*C. THROUGH SAID PASSAGE WHEREBY PYROLYTIC GRAPHITE IS FORMED ON SAID MEMBER AND SAID ENCLOSURE. 